chore: cleanup quality gate and frontend maintenance

[kilo-WATT]

Summary

This PR performs a small quality-gate and maintenance pass on the SleepLab frontend and repository tooling.

The goal was to improve developer workflow consistency, remove obsolete dependencies, and address lint/typecheck issues without changing application behavior.

Changes

Repository tooling

• Added/improved root-level quality gate commands:

  • build
  • lint
  • test
  • typecheck
  • test:frontend

• Added frontend Nx targets where needed to support consistent root-level execution.

Dependency cleanup

• Removed obsolete @types/react-router-dom.
• Continued using the types bundled with react-router-dom v7.

Frontend maintenance

• Replaced several any-based patterns with typed equivalents.
• Added typed handling for AHI chart click payloads.
• Converted RegisterRequest to a type alias where appropriate.
• Fixed minor lint/typecheck issues discovered during validation.

Component organization

• Extracted Oximeter import summary UI into:

  • frontend/src/components/OximeterImportSummary.tsx

• Extracted Oximeter file-selection helper into:

  • frontend/src/lib/oximeterFiles.ts

This reduces page-level complexity in Import.tsx while preserving existing behavior.

Documentation

• Added/updated README development command documentation.
• Documented common quality-gate commands for contributors.

Validation

Executed successfully:

npm run lint
npm run typecheck
npm run build
npm run test

npm run lint -w frontend
npm run test -w frontend
npm run build -w frontend
npm run typecheck -w frontend

Results:

• Frontend tests: passing
• Backend tests: passing (existing skipped tests unchanged)
• Frontend build: passing
• Typecheck: passing
• Lint: passing

Manual Verification

Validated on deployed LXC environment:

• Dashboard loads successfully
• Session detail page loads successfully
• Import page loads successfully
• Settings page loads successfully
• AHI chart navigation functions correctly

Not verified:

• Oximeter import workflow (no compatible device/files available for testing)

Notes

• No database migrations
• No importer behavior changes
• No authentication changes
• No AI workflow changes
• No Docker deployment changes
• No intentional user-facing behavior changes